Gas filtration media are typically used for air filtration and used in a number of applications such as ASHRAE filters (also known as pre-filters), HEPA (high efficiency particulate air) filters and ULPA (ultra low penetration air) filters. Such filters may for example be used for removing contaminations in clean rooms, for HVAC (heating, ventilating and air conditioning) systems, for protecting equipment, such as medical equipment, from contaminants.
A HEPA filter is typically defined as a filter which removes at least 99.97% of particles having a mean diameter of approximately 0.3 μm as measured by the DiOctyl Pthalate (DOP) test outlined in U.S. Military Standard MIL-STD-282 (1956), incorporated by reference herein, and U.S. Army document 136-300-175A (1965), incorporated by reference herein. Other equivalent standard test methods to generate sub-micron aerosol particles of size distribution centered on 0.3 micron mass-median diameter include, but are not limited to, British, French and European test protocols.
Moreover, the MIL-STD-282 test is also outlined in the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) Handbook of HVAC Systems, 1992, Chapter 25, pp. 25.3-25.5, incorporated by reference herein. In the ASHRAE Handbook at Chapter 25, various filters are classified in FIG. 4. In the preferred embodiment of the present invention, a high efficiency particulate air HEPA filter having 95% efficiency by the DOP MIL-STD-282 test is classified as Group IV in FIG. 4 of the ASHRAE Handbook.
Organic polymer based air filtration media based on PTFE are also available on the market and have been disclosed in a range of patent applications. However, PTFE media are typically rather fragile and hence require careful handling during manufacturing operation such as pleating or assembly or use and shaping as well as extensive support during use. For practical applications, PTFE does not have the strength to be used as a freestanding membrane.
U.S. Pat. No. 5,507,847 and WO 96/04063 discloses an ULPA filter having a PTFE membrane. The membrane is supported both upstream and downstream by one or more support materials. Other membrane materials than PTFE are suggested, but those membrane materials are not supported by experimental work and hence never reduced to practice.
U.S. Pat. No. 6,409,785 discloses a HEPA filter having two filter layers of which at least one is PTFE, wherein the second filter layer is intended to reduce the disadvantages of PTFE, which during the pleating operation tends to introduce cracks or tears in the PTFE membrane.
It should be observed that when the choice of material is discussed in patent applications in this field, polyethylene is sometimes suggested, but useable HEPA quality membranes of PE were until now never realized or reduced to practice in the art.